Moving element for a vibration exciter



O. J. ZAMPARO MOVING ELEMENT FOR A VIBRATION EXCITER Filed June 10, 1968 m 5 mm m w m WW r W 4 m nited States 3,536,942 MOVING ELEMENT FOR A VIBRATION EXCITER Oreste John Zamparo, Hamden, Conn., assignor to Textron Inc., Providence, RJL, a corporation of Delaware Filed June 10, 1968, Ser. No. 735,661 Int. Cl. H02k 33/18 US. Cl. 310-27 8 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a moving element for an electrodynamic vibration exciter.

A vibration exciter is composed, basically, of a moving element or armature suspended in a stationary housing and arranged to be reciprocated relative to the housing. Generally, the moving element consists of a table portion for communicating motion to a specimen, some form of coil arrangement for driving the element in a given direction, and a connecting portion for coupling the coil to the table portion. The element is supported with the coil disposed in an air gap formed in a field structure within the housing. The field structure is generally energized with direct current While the coil is supplied with an alternating current signal. Interaction between the alternating current and the field developed in the air gap imparts movement to the moving element.

One measure of exciter quality is the frequency range over which the exciter can be operated while satisfactory control is maintained over its output level. The low frequency limit is usually determined by the electronic equipment supplying power to the coil. The high frequency limit, however, is dependent on the lowest natural mechanical resonant frequency of the moving element since the electronic compensation systems presently available in the equipment supplying power to the coil cannot cope with the sharp changes in output level of the exciter when the resonant point is reached.

It has been found that the lowest mechanical resonant frequency of the moving element is, in part, a function of the materials employed in its construction. More specifically, the natural resonant frequency will be a function of the ratio of Youngs modulus of elasticity relative to the density of the particular material. This ratio is sometimes referred to as the specific stiffness. The higher the ratio, the higher the lowest natural resonant frequency.

Heretofore, moving elements generally have been con structed from various aluminum and magnesium alloys. Such alloys have a specific stiffness in the neighborhood of (102 to 104) 10 inches. Various steels and other structural metals have a specific stiffness of the same order of magnitude. Because of its substantially higher specific stiffness, well above 550x10 inches, consideration has been given to the use of beryllium. However, due to the extreme difiiculty in forming special shapes from beryllium, the cost of producing a moving element entirely from beryllium is presently prohibitive.

It is, therefore, an object of the present invention to provide a moving element having a higher lowest natural mechanical resonant frequency than that which would be obtained if the same moving element were fabricated from a material having a specific stiffness of the order of that of either a magnesium alloy or an aluminum alloy.

In essence, the invention makes use of beryllium in its least expensive form for certain critical portions of a moving element where it will have a significant effect upon the lowest natural resonant frequency. In accordance with the invention, there is provided a moving element for an electrodynamic vibration exciter comprising a table portion for communicating motion to a specimen, coil means for driving the element in a given direction, and a connecting portion for coupling the coil means to the table portion, the table portion being formed from a metal material having a specific stiffness of the order of that of magnesium alloys and aluminum alloys, while at least a major portion of the connecting portion is formed from a material having a substantially greater specific stiffness such as, for example, beryllium, the connecting portion being joined to both the coil means and the table portion for communicating movement from the coil means to the table portion in the given direction. More specifically, the connecting portion may comprise a plurality of flat plate members formed from sheet stock of beryllium, the members being oriented with their planes parallel to the given direction of movement of the element.

The invention will be better understood after reading the following detailed description of a presently preferred embodiment thereof with reference to the appended drawings in which:

FIG. 1 is a perspective view of a moving element constructed in accordance with the invention;

FIG. 2 is a sectional view taken along the line 22 in FIG. 1; and

FIG. 3 is a plan view of the lower or bottom surface of the table portion of FIG. 1.

Reference should now be had to the drawings wherein the same reference numerals are used throughout the several figures to designate the same or similar parts.

The table portion of the moving element is designated generally by the numeral 10. It is in the form of a disk provided on its upper surface with a plurality of equally spaced threaded bores 11 around the periphery in which steel inserts, not shown, are usually installed for permitting a specimen to be attached thereto. A bore 12 is provided at the center of the table portion 10 for insertion of a suitable transducer such as an accelerometer. The underside of the table portion 10, best seen in FIG. 3, is provided with a plurality of equi-angularly spaced radial channels 13 intersecting a central counterbore 14. The counterbore 14 is arranged to receive the upper end of the tube 15. The tube 15 is provided with a plurality of equi-angularly spaced longitudinal channels 16, A plurality of flat plate members 17 are fitted into the channels in the bottom of the table portion 10 and in the tube 15, as shown. Coil means 18 is disposed around the lower portions of the plate members 17, as best seen in FIGS. 1 and 2. Appropriate means, not shown, are provided for making electrical connection through flexible conductors to the coil 18.

The table portion 10 in the embodiment being described may be formed from a magnesium alloy. Satisfactory results have been obtained with AZ92A casting alloy having the general composition: 9A1 2Zn 0.1Mn 88.9Mg, and with AZ3lB (extruded and plate) having the general composition: 3A1 lZn 96Mg. The tube 15 may be formed from the same magnesium alloy as the table portion 10. Generally, the tube 15 will be produced by an extrusion process while the table portion 10 may be cast.

The plate members 17, or webs, may be formed from beryllium identified as PR-20 sheet stock manufactured by the Beryllium Corporation, of Reading, Pa. This is rolled HP20 Which has the general composition of at least 98% beryllium and no more than 25% beryllium oxide with 1.8% being typical.

In constructing the moving element herein described, the flat plate members 17 are adhesively bonded within the various channels 13 and 16 by means of an epoxy adhesive. A suitable adhesive is a thermosetting liquid such as the modified epoxy resin base product sold by Minnesota Mining and Manufacturing Company and identified as 3M Adhesive EC-2186. It will be found satisfactory to form the coil member 18 on a mandrel and transfer same to the edges of the plates 17 whereupon the self-supporting coil can be bonded to the beryllium plates by the same epoxy resin referred to above.

It will be seen that the table portion 10 has a flat specimen mounting surface, a circular periphery, and a surface opposite the mounting surface which is parallel thereto, i.e., its bottom surface. The flat plate members 17 intersect the table portion normal to the undersurface. The plates 17 lie in planes intersecting the central axis which is also normal to the undersurface of the table portion 10.

As best seen in FIG. 2, the tube 15 is disposed concentric with the central axis of the moving element.

For purpose of comparison, two structures were constructed, one with all parts but the coil formed from the same magnesium alloy, and the other formed from both magnesium alloy and beryllium as described above. The table portion was approximately of an inch thick and 2% inches in diameter. The plates 17 had a length of about 8 /2 inches and a thickness of about $4 inch. The tube 15 had a diameter of about W of an inch. In actual tests it was found that the lowest natural frequency of the structure having the foregoing dimensions when made completely of the magnesium alloy was about 6,200 hertz. When the webs or plate members 17 were formed from beryllium the lowest natural frequency was about 8,600 hertz. These figures are presented only as examples to provide some indication as to the significant improvement that can be obtained by the construction in accordance with the present invention.

It will be understood that the table portion or the tube portion or both may be constructed from an aluminum alloy or other metal material having a compar able specific stiffness rather than a magnesium alloy.

The invention has been described with reference to a specific embodiment thereof. It will be understood, however, by those skilled in the art that changes may be made therein without departing from the true spirit of the invention.

What is claimed is:

1. A moving element for an electrodynamic vibration exciter comprising a table portion for communicating motion to a specimen, coil means for driving the element in a given direction, and a connecting portion for coupling said coil means to said table portion, said table portion being formed from a metal alloy selected from the group consisting of magnesium alloys and aluminum alloys, while said connecting portion comprises a plurality of flat plate members formed from sheet stock of beryllium, said members being oriented with their planes parallel to said given direction and joined to both said coil means and said table portion for communicating movement from the coil means to the table portion in said given direction.

2. A moving element according to claim 1, wherein said flat plate members are joined to said table portion by a resinous adhesive.

3. A moving element according to claim 1, wherein said table portion is in the form of a disk having a flat specimen mounting surface, a circular periphery, and a surface opposite said mounting surface which is parallel thereto; and said fiat plate members intersect said table portion normal to said opposite surface thereof and disposed along angularly spaced planes radiatingfrom a central axis which is normal to said opposite surface.

4. A moving element according to claim 3, wherein said flat plate members radiate from a tube to which they are bonded, said tube being disposed concentric with said central axis with one end joined to said table portion.

5. A moving element according to claim 4, wherein said tube is formed from a metal alloy selected from the group consisting of magnesium alloys and aluminum alloys.

6. A moving element according to claim 5, wherein said fiat plate members are recessed into channels formed in both said table portion and said tube.

7. A moving element for an electrodynamic vibration exciter comprising a table portion for communicating motion to a specimen, coil means for driving the element in a given direction, and a connecting portion for coupling said coil means to said table portion, said table portion being formed from a metal alloy selected from the group consisting of magnesium alloys and aluminum alloys, while at least a major portion of said connecting portion is formed from beryllium, said connecting portion being joined to both said coil means and said table portion interposed therebetween for corm'nunicating movement from the coil means to the table portion in said given direction.

8. A moving element for an electrodynamic vibration exciter comprising a table portion for communicating motion to a specimen, coil means for driving the element in a given direction, and a connecting portion for coupling said coil means to said table portion, said table portion being formed from a metal material having a specific stiffness of the order of that of magnesium alloys and aluminum alloys, while at least a major portion of said connecting portion is formed from a metal material having a substantially greater specific stiffness, said connecting portion being joined to both said coil means and said table portion interposed therebetween for communicating movement from the coil means to the table portion in said given direction.

References Cited UNITED STATES PATENTS 3,417,268 12/1968 Lace 310-27 3,018,541 1/1962 Hunt et a1. 310-27 XR 2,751,512 6/1956 Reen et al 73--71.6 XR 2,632,791 3/1953 Side 17541.5 1,759,632 5/1930 Scheuch 15O XR 3,234,782 2/1966 Grootenhuis 310-27 XR 3,296,107 1/1967 Nakamura 2:04-65 3,349,597 10/1967 Gross 75150 XR 3,075,100 1/1963 Ethomson 310-27 3,106,653 10/1963 Fowler 31027 2,810,842 10/1957 Lewis 31027 MILTON O. HIRSHFI ELD, Primary Examiner B. A. REYNOLDS, Assistant Examiner US. Cl. X.R.

mg UNITED STATES PATENT OI FICE CERTIFICATE OF CORRECTION October 27, 1970 Patent: NO. 3I536'942 Dated Inventor(s) ORESTE JOHN ZAMPARO It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 3, "25%" shcpuld read -2% SELLZ n1 DEC 29 L- 0 (SEAL) Ali/est:

Edward M. Fletcher, Ir.

Attesu'ng Officer m. L Omission of Pat! 

